1. Field of the Invention
Novel liquid crystal display cells having liquid crystal molecules that are positioned in a vertical or substantially vertical alignment are described herein. More particularly, novel liquid crystal displays formed from substantially homogeneous alignment layers disposed on transparent electrodes with the alignment layers being made of a substantially homogeneous fluorinated material are described. Also described are methods for manufacturing the liquid crystal display cells.
2. Description of the Related Art
In general, a liquid crystal display (LCD) includes a liquid crystal layer containing liquids crystal molecules and a pair of substrates on either side of the liquid crystal layer. When the alignment of the liquid crystal molecules is changed, the optical refractive index of the liquid crystal layer is also changed. By utilizing the change in the refractive index, the LCD performs the display. Accordingly, it is important that the liquid crystal molecules are arranged as regularly as possible in the initial state.
Interactions between the liquid crystal molecules and the surfaces of the substrates between which the liquid crystal layer is formed frequently determine the orientation of the liquid crystal molecules. To regulate the orientation of the liquid crystal molecules, an alignment layer can be applied to each of the surfaces of the substrates which face the liquid molecules. There are generally two types of materials employed to form the alignment layer for regulating the alignment of the liquid crystal molecules. One type of materials used to form the alignment layer are inorganic materials such as, for example, oxides, inorganic silane, metals and metal complexes. The other type of materials are organic materials, e.g., a polyimide. Typically, the desired material is applied to the substrate, dried and cured to form the alignment layer.
The liquid crystal molecules are then aligned by inducing anisotropy on the surface of the alignment layer. Methods for introducing anisotropy on the surface include, for example, (1) rubbing as disclosed in deGennes et al., xe2x80x9cThe Physics of Liquid Crystalsxe2x80x9d, Clarendon Press (1993), pp. 109 and 161; (2) stretching a polymer as disclosed in Aoyama et al., xe2x80x9cMol. Cryst. Liq. Cryst.xe2x80x9d (1981) pp. 72 and 127; (3) applying a Langmuir-Blodgett film as disclosed in Ikemo et al., Jpn. J. Appl. Phys., 27, L475 (1998); (4) providing a grating structure produced by microlithography as disclosed in Nakamura et al., J. Appl. Phys., 52, 210 (1981); (5) oblique angle deposition of SiOx as disclosed in Ienuing, Appl. Phys. Lett., 21, 173 (1982); and, (6) polarized UV radiation of polymer films as disclosed in Schadt et al., J. Appl. Phys., 31, 2155 (1992). These methods, except that of example (1), are impractical due to their complexity and/or cost for mass production.
Currently, the method most often employed in aligning the liquid crystal molecules in a liquid crystal display cell is a mechanical rubbing treatment. This method involves unidirectionally rubbing the surface of the alignment layer with, for example, a velvet or burnishing cloth, to generate a proper pretilt angle, e.g., from 1xc2x0 to 5xc2x0 from the substrate surface, of the liquid crystal molecules in the liquid crystal layer which are in contact with the alignment layer are all equal to each other. One such example of a liquid crystal display device formed from this method is U.S. Pat. No. 5,795,629 which describes a display device formed by (1) applying a non-homogeneous alignment layer to a substrate where the alignment layer is formed from a mixture of two or more polymers with one of the polymers containing fluorine or siloxane; (2) rubbing the alignment layer in one direction with a nylon cloth; (3) assembling the substrates together; and (4) filling the space between the substrates with a nematic liquid crystal to form a liquid crystal display device.
Several drawbacks are associated with this rubbing method. Firstly, debris is left on the surface of the alignment layer by the cloth during the rubbing process which can damage an otherwise clean room environment. Secondly, the vigorous rubbing may damage, e.g. scratch, the structure of the liquid crystal display cell. Thirdly, electrostatic discharging may influence the electronic circuitry below the alignment layer. Fourthly, stability and consistency of pretilt angles are difficult to achieve and control. Finally, rubbing, being a macroscopic process, does not readily lend itself to aligning liquid crystal molecules in different directions at the spatial size of a pixel, which is usually of the order of a hundred micrometers. This local alignment is desirable to obtain a multidomain structure, which significantly increases the angle of viewing of the display. Thus, the limited angle of viewing of current liquid crystal display cells is one of the limitations of this technology.
It would be desirable to provide a liquid crystal display cell having liquid crystal molecules that are positioned in a vertical or a substantially vertical alignment produced by a non-contact method, i.e., nonrubbing method, thereby overcoming the deficiencies of the prior art discussed above.
Novel liquid crystal displays formed from liquid crystals display cells for use in, e.g., television sets or personal computers, have been discovered. The novel liquid crystal display cells having liquid crystal molecules positioned in a vertical or a substantially vertical alignment include at least two substantially homogeneous alignment layers disposed on transparent electrodes with each alignment layer formed from a substantially homogeneous fluorinated material, a liquid crystal layer of liquid crystal molecules disposed between the alignment layers, and a means for applying voltage across the transparent electrodes.
In one embodiment, liquid crystal display cells used to make the liquid crystal display include at least a first and second substrate, transparent electrodes applied to opposite surfaces of the first and second substrates, an adhesion layer applied to a portion of each surface of the transparent electrodes, a fluorinated alignment layer applied to a portion of the adhesion layers, a liquid crystal layer of liquid crystal molecules disposed between the fluorinated alignment layers and a means for applying voltage across the transparent electrodes.
In another embodiment, a liquid crystal display cell is provided which includes a first and second substrate, transparent electrodes applied to opposite surfaces of the first and second substrates, a substantially homogeneous layer applied to a portion of each surface of the transparent electrodes, the exposed surface of the substantially homogeneous layer being fluorinated to provide a substantially homogeneous fluorinated alignment layer, a liquid crystal layer of liquid crystal molecules disposed between the alignment layers and a means for applying voltage across the transparent electrodes.
Methods for making the liquid crystal display cells possessing liquid crystal molecules positioned in a vertical alignment have also been discovered. In one method, a transparent electrode is applied to a portion of a surface of a first and second substrate, an adhesion layer is applied to a portion of each surface of the transparent electrodes, a fluorinated alignment layer is applied to a portion of the adhesion layers, and a liquid crystal layer of liquid crystal molecules is disposed between the alignment layers of the two substrates.
In a second method, a transparent electrode is applied to a portion of a surface of a first and second substrate, a substantially homogeneous layer is applied to a portion of each surface of the transparent electrodes, the substantially homogeneous layers are then fluorinated to provide a substantially homogeneous fluorinated alignment layer, and a liquid crystal layer of liquid crystal molecules is disposed between the alignment layers.
A method for providing a liquid crystal display cell having liquid crystal molecules positioned in a substantially vertical alignment has also been discovered. In the method the surface of at least one of the alignment layers in the foregoing liquid crystal display cells is exposed to an energy beam such as an ion beam to adjust the pretilt angle of the liquid crystal molecules vertically aligned with respect to the alignment surface to provide a liquid crystal display cell having liquid crystal molecules positioned in a tilted vertical alignment. The advantages of employing this method are (1) no contact is necessary for the alignment; (2) a low energy beam ensures that only the surface of the substantially homogeneous alignment layers are affected such that the number of radicals induced by broken bond are at a minimum which will avoid a charge build up when a voltage is applied across the liquid crystal display cell; (3) a large area uniform and parallel beams can be readily obtained; and, (4) energy beams are well known in the electronics manufacturing community such that compatibility with a clean room environment can be readily obtained.
The liquid crystal display cells and methods described herein advantageously permit the liquid crystal molecules to be positioned in a vertical or a tilted vertical alignment.